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Dean Harman is a professor of chemistry at the University of Virginia, where he has been honored with several teaching awards. He heads Harman Research Group, which specializes in the novel organic transformations made possible by electron-rich metal centers such as Os(II), RE(I), AND W(0). He holds a Ph.D. from Stanford University.

Gordon Yee is an associate professor of chemistry at Virginia Tech in Blacksburg, VA. He received his Ph.D. from Stanford University and completed postdoctoral work at DuPont. A widely published author, Professor Yee studies molecule-based magnetism.

Tarek Sammakia is a Professor of Chemistry at the University of Colorado at Boulder where he teaches organic chemistry to undergraduate and graduate students. He received his Ph.D. from Yale University and carried out postdoctoral research at Harvard University. He has received several national awards for his work in synthetic and mechanistic organic chemistry.

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We have seen how a very polarized bond between carbon and another atom can lead to a substitution reaction on carbon. Specifically, by having that bond be very polarized, it renders the carbon electrophyllic and that carbon therefore can be attacked by a nucleophile, generating the net substituted product, if we have the electronegative atom come off as a leaving group.
Now, let's look at another example of a substitution reaction and we're going to see that there's a bit of an unexpected outcome in this case. Let's suppose that the reaction we're trying to do is to take this chlorohydrocarbon and react it with sodium methoxide. So we've got our electrophyllic carbon, we've got a good nucleophile here and the reaction we're anticipating is the formation of a bond between the nucleophile and the electrophile, where the chloride comes off as a leaving group. Again I remind you that this is a very polarized bond and that pair of electrons can come off with the chloride.
So maybe this is the product we expect and maybe a little bit of this product is actually made, but it turns out to be the minor product. The major product in this reaction is something that we might have not expected, it's an alkene, characterized by a carbon-carbon double-bond. Remember that tells us it's an alkene. And we also get HCl. Now, this is an example of a reaction called an elimination reaction and let's see why. If we look at the overall reaction what happens here? We have eliminated a neutral molecule, hydrochloric acid, from our starting material in this case to make that carbon-carbon double-bond. So, in that sense, by eliminating this neutral molecule, it's an example again of an elimination reaction. That doesn't tell us anything about how it happens and, of course, that's what we're really trying to probe in this unit, is how do reactions occur and how can we link what we know about molecular structure to the mechanism of a reaction? So let's see if we can figure out where this came from.
Now, we know that, by this bond being very polarized, it's possible that this leaving group could simply come off first. In other words, it's possible that this bond could undergo heterolytic cleavage to give us a carbocation and chloride. And if that were to happen first, we would expect our nucleophile to combine with our electrophile and give us that substitution product. But what if that nucleophile attacks this hydrogen instead? In other words, what if a Brønsted acid base reaction occurred rather than the Lewis acid base reaction that we were expecting? In that case, follow through what happens with electrons now, if this attacks this hydrogen, the pair of electrons in this bond would be left behind with this carbon. And normally that's CH bond would not acidic at all, but understand that it's next to a very electron-deficient thing now, this carbocation, making this a much stronger Brønsted acid than we normally would expect for a CH bond. So if this thing pulls off the hydrogen, that pair of electrons then can move over and help out this carbon, making a pi bond in the process, and that gives us our final product. So again, recognize the elements here are exactly the same types of things we've been talking about before, polarized bond, heterolytic cleavage, a carbocation intermediate that makes a Lewis acid, that, by inductive effects, makes a good Brønsted acid. We get a Brønsted acid base reaction here, we're just pulling off a proton. That pair of electrons moves over to make the double-bond. So we can make sense out of this reaction even though we've never seen it before. The steps are starting to feel somewhat familiar to you. Even though you've never seen an elimination reaction, per say, the elements that go into it are all things that we've talked about before.
Now, let's look at something else. Let's suppose that we took the product of that reaction, the alkene, and combined that with hydrochloric acid. What we'll find is that we get out the starting material of that last reaction. In other words, what we've done is, in a sense, the reverse of the reaction we just talked about, the reverse of an elimination, which is an addition reaction. Why is it called an addition reaction? We have overall added a hydrogen and chlorine, the elements of this neutral molecule, across this carbon-carbon double-bond in this case. So by adding this molecule to this one to come up with a new molecule, it's classified as an addition reaction. And notice that we've added a hydrogen and chloride, an electrophile and a nucleophile, across that double bond here. So again, this would be an addition reaction and again, that's the reverse of the idea of an elimination reaction.
Now, how does this actually happen? Well, we're going to get a big clue by thinking about what we just talked about, because of a very important principle, that if there's a mechanism that gets us from a to b, provided that b is not so low energy that it can't ever get back to a, then there's going to be a mechanism that takes b back to a that looks like exactly the reverse of the mechanism that got us from a to b. This is called the principle of microscopic reversibility. It just simply says that an energy pathway that gets us from a to b also will exist to get us from b back to a by the exact same sequence of steps, whatever they are, but just in reverse. Now, we have to be a little bit careful here, because since the two reactions I'm talking about are not exactly the same - they're the same reactants and products, but the reagent conditions are different. In other words, in one case I'm using acid and the other case I did it in the presence of base. So the energy on this diagram will be a little different, so I just want to be careful here. But the idea is the most important, that the same way that we went from our reactant to our product, we can do the same thing backwards. So when we're worried about an addition reaction, it's going to be the reverse idea of an elimination reaction.
So what would that look like? I want to remind you that, in an alkene, there is a pi bond, and that that corresponds to electron density above and below the plane of the molecule. Now, we've seen before a pair of electrons attacking a proton, but we haven't seen a bond act as a base before. So that's a new idea, but just in the same way that the lone pair on a nitrogen can act as a base, so too can the pair of electrons involved in a pi bond act as a base. It's not a very good one, but if you have a strong enough acid, it gets its way. And you can form a new bond now between that pair of electrons and this hydrogen and, in the process, this chloride comes out as Cl^-, taking that pair of electrons with it. That gives us a carbocation and that carbocation we recognize as a Lewis acid, like a borane again, with an empty p orbital. And that Lewis acid can be attacked by the chloride nucleophile to make the carbon-chlorine bond. So the steps involved in this addition reaction are, in fact, very similar to steps we've seen before. Once again, we have a very polarized bond making this very reactive as a Brønsted acid in this case. It is attacked readily by the pair of electrons in this double bond, so that's a Brønsted acid-base reaction essentially, generating a Lewis acid. That Lewis acid can combine with a nucleophile to give us our final product, the net reaction being an addition of hydrogen and chlorine across the carbon-carbon double bond.
So overall, let's take stock of what we've done. We've seen now our first examples of mechanisms for substitution reactions, elimination reactions and addition reactions. Particularly I've shown you examples of organic reactions, but I remind you that this, in fact, applies to much more than carbon. This applies to anything in the periodic table. The basic idea that, by understanding the structure and by looking at where bonds are polarized, looking for reactivity in those positions, we've seen that much of the types of reactions that we've looked at are explained in terms of Lewis acids interacting with bases. In other words, an electrophile interacting with a nucleophile. In simple terms, the electron-rich portion of a molecule reacts with the electron-deficient portion of a molecule. So although we can't look at every reaction yet and predict exactly what's going to happen, we can start to make sense out of different mechanisms that we're presented with and come up with at least a good level of understanding, a rationalization of why certain reactions occur.
Introductions to Organic Reactions
Introduction to Electrophiles and Nucleophiles
Elimination Reactions Page [1 of 2]

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